Martensitic stainless steel with excellent weld characteristics, and martensitic stainless steel material

ABSTRACT

This martensitic stainless steel contains, in terms of percent by mass: C: 0.003% to 0.03%; Si: 0.01% to 1.0%; Mn: 3.0% to 6.0%; P: 0.05% or less; S: 0.003% or less; Ni: 1.0% to 3.0%; Cr: 15.0% to 18.0%; Mo: 0% to 1.0%; Cu: 0% to 2.0%; Ti: 0% to 0.05%; N: 0.05% or less; Al: 0.001% to 0.1%; and O: 0.005% or less, with a remainder being Fe and inevitable impurities, wherein a total amount of C and N is in a range of 0.060% or less, γ max  represented by the formula 1 is in a range of 80 or more, and γ pot  represented by the formula 2 is in a range of 60 to 90. 
       γ max =420×C %+470×N %+23×Ni %+9×Cu %+7×Mn %−11.5×Cr %−11.5×Si %−52×Al %+189  Formula 1
 
       γ pot =700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−74.4×Ti %−37.2×Al %+63.2  Formula 2
 
     Here, C %, N %, Ni %, Cu %, Mn %, Cr %, Si %, A1%, Mo %, and Ti % represent the contents (mass %) of the respective elements.

TECHNICAL FIELD

The present invention relates to a martensitic stainless steel suitably used for portions that are needed to be welded in welded structures such as construction structures, ship structures, and the like, and the present invention also relates to a low-cost martensitic stainless steel material which is manufactured using the martensitic stainless steel, and which is excellent in impact characteristics and corrosion resistance of a base material and welded portions, and the cost is low since the Ni content is redcued in the martensitic stainless steel material.

The present application claims priority on Japanese Patent Application No. 2010-60048 filed on Mar. 17, 2010, the content of which is incorporated herein by reference.

BACKGROUND ART

The martensitic stainless steel is widely used for things such as blades, springs, brake discs, and the like since the strength can be easily enhanced through a quenching thermal treatment. However, the martensitic stainless steel has a low toughness and poor weldability; and therefore, the martensitic stainless steel is not used for welded structures.

Meanwhile, a steel material having improved toughness, weldability, and corrosion resistance is developed by reducing the content of C and adding approximately 3% or more of Ni to a steel containing 13% to 17% of Cr, and the steel material is used for water wheel runners for hydroelectric power generation or steel pipes for oil well (for example, Patent Documents 1 to 4).

However, even in the improved martensitic stainless steel as described above, tempering resistance is extremely large. Therefore, a problem still remains in that thermal treatment facility capacities are impaired since a long-term treatment or the like is required in a tempering thermal treatment for tempering the characteristics of a final product, and the manufacturing costs are high.

Therefore, a martensitic stainless steel for which a thermal treatment for tempering is not required or manufacturing conditions for which a dehydrogenation treatment is not required are studied, and Patent Document 4 is disclosed which aims to obtain a martensitic single-phase structure, and Patent Document 5 is disclosed which has a multiphase structure mainly including a martensite phase and including a ferrite phase or a residual austenite phase.

As disclosed in Patent Document 4, in the majority of martensitic stainless steels, the amount of Cr is in a range of 11% to 15%, the corrosion resistance is poor compared to a ferrite stainless steel such as SUS430, and there are cases in which rusting (occurrence of rusts) is caused even in an indoor environment. Therefore, in order to obtain excellent corrosion resistance, it is necessary to add Mo or increase the amount of Cr.

In addition, Patent Document 5 discloses that 15% or more of Cr or 1% or more of Mo is preferably included in order to enhance corrosion resistance. However, the martensitic stainless steel of Patent Document 5 has a metallic structure mainly including a martensite phase that includes a ferrite phase, the hot workability is not favorable, and there was a problem in that the manufacturing yield of the steel material frequently degraded. In addition, in order to secure mechanical characteristics, it is necessary to add austenite-forming elements at amounts that commensurate with the increment in amounts of Cr and Mo; and thereby, an increase in the alloy costs is caused.

That is, as the steel that can favorably maintain the characteristics of a base material and welded portions, a steel containing a large amount of Ni is in practical use. However, there was no practical steel which had favorable hot workability, corrosion resistance equivalent to that of SUS430 in a base material and welded portions, and excellent mechanical characteristics, and in which the amount of Ni was reduced so as to be inexpensive.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Unexamined Patent Application, First     Publication No. H06-306549 -   Patent Document 2: Japanese Unexamined Patent Application, First     Publication No. H06-306551 -   Patent Document 3: Japanese Unexamined Patent Application, First     Publication No. H02-243739 -   Patent Document 4: Japanese Unexamined Patent Application, First     Publication No. H02-243740 -   Patent Document 5: Japanese Unexamined Patent Application, First     Publication No. 2001-279392

Non Patent Document

-   Non-Patent Document 1: Current Advances in Materials and Processes,     Vol. 3 (1990), 1840

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In consideration of the above-described problems, the present inventors address an object of the invention of clarifying the component system and metallic structure of inexpensive martensitic stainless steel having favorable hot workability, mechanical characteristics, and corrosion resistance equivalent to that of SUS430, and developing a practical steel material.

Means for Solving the Problems

C, N, Mn, Cu, Co, or the like is considered as an element that can replace Ni; however, with regard to the above-described martensitic stainless steel, there is a few publications in relation to a steel containing large amounts of Mn, Cu, and Co. As an example, Non-Patent Document 1 shows an example in which a high-purity stainless steel containing 17% of Cr is used as the base, and Ni or Mn is added. However, Non-Patent Document 1 does not disclose an example in which both of Ni and Mn are added, and does not consider corrosion resistance either.

Meanwhile, generally, Mn is an element that degrades corrosion resistance. Therefore, there are only a few examples in which active addition of Mn is attempted to a martensitic stainless steel having poor corrosion resistance compared to an ordinary stainless steel. In the case where the amount of C is increased and, at the same time, the amount of Mn is increased, whether or not a desired corrosion resistance can be obtained is still under question. Therefore, based on technical aspects or experience thus far obtained, it was impossible to employ a method of adjusting the above-described alloy elements in order to develop a practical steel material that can secure excellent hot workability and mechanical characteristics as well as corrosion resistance.

Regarding a steel in which a steel containing 16% of Cr and 2% of Ni is used as the base and 2% or more of Mn is added, the inventors studied in detail the influence of the component elements and the metallic structure of a steel material on a variety of the above-described characteristics. As a result, it was found that the amount of Ni, whose price widely varies, can be suppressed, and both of toughness and corrosion resistance of welded portions can be satisfied by setting the amounts of Cr, Ni, Mn, and other elements described below in predetermined ranges. Furthermore, it was found that the mechanical characteristics of a base material can be secured by setting the phase fractions in a steel material even when thermal treatments of quenching and tempering which were required in the related art are not carried out. Based on the above findings, the invention has been completed.

The features of the invention are as follows.

(1) Martensitic stainless steel with excellent weld characteristics according to an aspect of the invention contains, in terms of percent by mass, C: 0.003% to 0.03%, Si: 0.01% to 1.0%, Mn: 3.0% to 6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0% to 3.0%, Cr: 15.0% to 18.0%, Mo: 0% to 1.0%, Cu: 0% to 2.0%, Ti: 0% to 0.05%, N: 0.05% or less, Al: 0.001% to 0.1%, and O: 0.005% or less with a remainder being Fe and inevitable impurities. A total amount of C and N is in a range of 0.060% or less, γ_(max) represented by the formula 1 is in a range of 80 or more, and γ_(pot) represented by the formula 2 is in a range of 60 to 90.

γ_(max)=420×C %+470×N %+23×Ni %+9×Cu %+7×Mn %−11.5×Cr %−11.5×Si %−52×Al %+189  Formula 1

γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−74.4×Ti %−37.2×Al %+63.2  Formula 2

Here, C %, N %, Ni %, Cu %, Mn %, Cr %, Si %, Al %, Mo %, and Ti % represent the contents (mass %) of the respective elements.

(2) The martensitic stainless steel with excellent weld characteristics according to the aspect of the invention described in the above (1) may further contain Nb, and γ_(pot) that is calculated by the formula 3 instead of the formula 2 may be in a range of 60 to 90.

γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−3.1×Nb %−74.4×Ti %−37.2×Al %+63.2  Formula 3

Here, C %, N %, Mn %, Cu %, Ni %, Si %, Cr %, Mo %, Nb %, Ti %, and Al % represent the content (mass %) of the respective elements.

(3) The martensitic stainless steel with excellent weld characteristics according to the aspect of the invention described in the above (1) or (2) may further contain either one or both of V and W.

(4) The martensitic stainless steel with excellent weld characteristics according to the aspect of the invention described in any one of the above (1) to (3) may further contain Co.

(5) The martensitic stainless steel with excellent weld characteristics according to the aspect of the invention described in any one of the above (1) to (4) may further contain one or more selected from B, Ca, Mg, and REM.

(6) A martensitic stainless steel material according to an aspect of the invention has a composition described in any one of the above (1) to (5), and has a structure including 5% to 30% of a ferrite phase and 0% to 20% of a residual austenite phase with a remainder being a martensite phase.

(7) The martensitic stainless steel material according to the aspect of the invention described in the above (6) may have a yield strength in a range of 400 MPa to 800 MPa.

Effects of the Invention

The martensite steel having the composition of the aspect of the invention exhibits an effect of excellent toughness and excellent corrosion resistance of welded portions. In addition, according to the aspect of the invention, it is possible to provide a inexpensive martensitic stainless steel material that can be used for large-scale welded structures such as construction structures, ship structures, and the like. In addition, since desired characteristics can be obtained even when long-term thermal treatments of quenching and termpering are not carried out, mass productivity can be improved. Therefore, the aspect of the invention can significantly contribute to the industry.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, firstly, the reasons why the chemical composition of the martensitic stainless steel of the present embodiment is limited will be described. Meanwhile, the unit of the contents of the respective components in the following is mass %.

C is included at a content of 0.003% or more in order to secure the strength of a steel. However, in the case where more than 0.03% of C is included, the strength increases more than necessary, and corrosion resistance and toughness deteriorate in welded portions. Therefore, the content of C is limited to 0.003% to 0.03%. The content of C is preferably in a range of 0.005% to 0.025%.

Si is added at a content of 0.01% or more for deoxidization. However, in the case where more than 1.0% of Si is added, toughness deteriorates. Therefore, the upper limit of the content of Si is limited to 1.0%. The content of Si is preferably in a range of 0.2% to 0.5%.

Mn is added at a content of 3.0% or more in order to improve the toughness of welded portions. However, an increase in the content of Mn deteriorates corrosion resistance. In the steel of the embodiment, the content of Mn, γ_(max), γ_(pot), and the fraction of a ferrite phase in a steel material of the embodiment which will be described below have a close relationship, and deterioration of the corrosion resistance due to an increase in the content of Mn is suppressed by controlling a metallic structure. However, in the case where more than 6.0% of Mn is included, desired corrosion resistance cannot be secured. Therefore, the upper limit of the content of Mn is limited to 6.0%. The content of Mn is preferably in a range of 3.5% to 5.5%.

Since P deteriorates hot workability and toughness, the content of P is limited to 0.05% or less. The content of P is preferably in a range of 0.03% or less. In addition, P is an element that is inevitably included in a steel, and the smaller the content thereof, the more preferable. However, extreme reduction causes an increase in the costs; and therefore, generally, P is inevitably included at a content of approximately 0.005% or more.

Since S deteriorates hot workability, toughness, and corrosion resistance, the content of S is limited to 0.003% or less. The content of S is preferably in a range of 0.001% or less. In addition, S is also an element that is inevitably included in a steel, and the smaller the content thereof, the more preferable. However, extreme reduction causes an increase in the costs; and therefore, generally, S is inevitably included at a content of approximately 0.0001% or more.

Ni stabilizes an austenite structure, and Ni improves corrosion resistance with respect to a variety of acids and, furthermore, Ni improves toughness. Therefore, Ni is included at a content of 1.0% or more. On the other hand, Ni is an expensive alloy, and the content of Ni is limited to 3.0% or less from the viewpoint of costs. The content of Ni is preferably in a range of 1.5% to 2.5%.

Cr is included at a content of 15.0% or more in order to secure basic corrosion resistance. On the other hand, in the case where more than 18.0% of Cr is included, toughness and corrosion resistance in welded portions are impaired. Therefore, the content of Cr is set to be in a range of 15.0% to 18.0%. The content of Cr is preferably in a range of 16% to 17%.

Mo is an element extremely effective for incrementally enhancing the corrosion resistance of a stainless steel, and Mo is an arbitrary component (selective component) that is included as necessary. Since Mo is an extremely expensive element, in the case where Mo is added to enhance corrosion resistance, the upper limit of the content of Mo is set to 1.0% or less from the viewpoint of costs. In the case where Mo is added, the content of Mo is preferably in a range of 0.1% to 0.5%.

Cu is an element having actions of incrementally enhancing corrosion resistance of a stainless steel with respect to acids and improving toughness, and Cu is an arbitrary component (selective component) that is included as necessary. In the case where more than 2.0% of Cu is included, the content of Cu exceeds the solid solubility such that eCu precipitates and embrittlement occurs. Therefore, in the case where Cu is included, the upper limit of the content of Cu is set to 2.0%. Cu has effects of stabilizing an austenite phase and improving toughness. In the case where Cu is included, the content of Cu is preferably in a range of 0.2% to 1.5%.

An extremely small amount of Ti forms oxides, nitrides, and sulfides, solidifies steel, and Ti is a grain refining element in a solidified and high-temperature-heated structure, and Ti is an arbitrary component (selective component) that is included as necessary. In the case where more than 0.05% of Ti is included, a ferrite phase is generated, and TiN is generated; and thereby, the toughness of a steel is impaired. Therefore, in the case where Ti is included, the upper limit of the content of Ti is set to be 0.05%. In the case where Ti is included, the content of Ti is preferably in a range of 0.003% to 0.020%.

N is included at a content of 0.01% or more in order to enhance the strength of a martensite phase. However, in the case where more than 0.05% of N is included, the strength is excessively increased; and thereby, toughness is deteriorated. Therefore, the content of N is limited to 0.05% or less. The content of N is preferably in a range of 0.01% to 0.04%.

Al is an element important for deoxidization of a steel, and Al is included with Si in order to reduce oxygen in a steel. It is essential to reduce the content of oxygen so as to secure toughness; and therefore, it is necessary to include 0.001% or more of Al. On the other hand, Al is an element that increases an amount of a ferrite phase, and, in the case where Al is added excessively, toughness is impaired. In the case where the content of Al exceeds 0.1%, toughness becomes greatly degraded. Therefore, the upper limit of the content of Al is set to be 0.1%. The content of Al is preferably in a range of 0.01% to 0.05%.

O is an element that composes oxides which are representative non-metallic inclusions, and O is inevitably included in a steel. Therefore, the smaller the content of O, the more preferable. However, extreme reduction causes an increase in the costs. Therefore, generally, O is inevitably included at a content of approximately 0.001% or more. On the other hand, in the case where an excessive amount of O is included, toughness is impaired. In addition, when coarse cluster-shaped oxides are generated, the oxides cause surface cracking. Therefore, the upper limit of the content of O is set to 0.005%.

The sum (C+N) of the contents of C and N has a relationship with the strength of a steel. In the case where the sum (C+N) of the contents of C and N exceeds 0.060%, the strength enhances excessively, and toughness is impaired. Therefore, the upper limit of the sum (C+N) of the contents of C and N is set to 0.060%. The sum (C+N) of the contents of C and N is preferably in a range of 0.015% to 0.050%.

γ_(max) represented by the following formula 1 is a computation formula that estimates the maximum value of the fraction of an austenite phase generated in a temperature range of 900° C. to 1000° C. The toughness of a steel can be enhanced by increasing the value of the γ_(max). In the embodiment, in the case where the value of γ_(max) is less than 80%, an amount of a ferrite phase becomes excessive large, and a ferrite band structure remains; and thereby, desired toughness cannot be secured. Therefore, γ_(max) is set to be in a range of 80% or more. γ_(max) is preferably in a range of 85% or more.

γ_(max)=420×C %+470×N %+23×Ni %+9×Cu %+7×Mn %−11.5×Cr %−11.5×Si %−52×Al %+189  Formula 1

Here, C %, N %, Ni %, Cu %, Mn %, Cr %, Si %, and Al % represent the contents (mass %) of the respective elements.

γ_(pot) represented by the following formula 2 is a computation formula that shows the fraction of a martensite phase in a cast state, and γ_(pot) also corresponds to the fraction of an austenite phase during hot working. In the embodiment, a range of γ_(pot) is determined in order to secure hot workability. When γ_(pot) increases, a fraction of a soft ferrite phase decreases excessively; and thereby, strains concentrate in the ferrite phase during hot working, and cracking is promoted. The upper limit of γ_(pot) is dependent on the content of Mn or the content of Si that influences hot workability. In the embodiment, in the case where γ_(pot) exceeds 90%, there is a problem in that the manufacturing yield of the steel material decreases greatly. Therefore, the upper limit of γ_(pot) is set to be 90%. On the other hand, in the case where γ_(pot) is less than 60%, C and N concentrate in a martensite phase which is generated in welded portions; and thereby, the martensite phase becomes hard and turns into a heterogeneous structure. In addition, since the corrosion resistance of the martensite phase in which alloying elements of C, N, Mn, and the like concentrate degrades, the lower limit of γ_(pot) is set to be 60%. γ_(pot) is preferably in a range of 65% to 85%.

γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−74.4×Ti %−37.2×Al %+63.2  Formula 2

Here, C %, N %, Ni %, Cu %, Mn %, Cr %, Si %, Al %, Mo %, and Ti % represent the contents (mass %) of the respective elements.

Next, the reasons why the arbitrary components (selective components) of the embodiment are limited will be described. The elements that will be described below are arbitrary components (selective components) that are added as necessary.

Nb is an element effective for miniaturizing crystal grains in a hot-rolled structure. Furthermore, Nb also has an action of enhancing corrosion resistance. Nitrides and carbides that Nb generates are generated during processes of hot working and a thermal treatment, and have an action of suppressing growth of crystal grains and strengthening a steel and a steel material. Therefore, Nb may be included at a content of 0.01% or more. On the other hand, when an excessive amount of Nb is added, Nb precipitates in the form of a non-dissolved precipitate during heating before hot rolling, and Nb impairs toughness. Therefore, the upper limit of the content of Nb is set to be 0.2%. In the case where Nb is included, the content of Nb is preferably in a range of 0.03% to 0.10%.

γ_(pot) represented by the following formula 3 is a computation formula that shows the fraction of a martensite phase in a cast state in the case where Nb is included, and γ_(pot) also corresponds to the fraction of an austenite phase during hot working In the case where Nb is included, γ_(pot) calculated using the formula 3 instead of the above formula 2 is set to be in a range of 60% to 90%, and the formula 3 includes the Nb element. Even in the case where Nb is included, γ_(pot) is preferably in a range of 65% to 85%.

γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−3.1×Nb %−74.4×Ti %−37.2×Al %+63.2  Formula 3

Here, C %, N %, Mn %, Cu %, Ni %, Si %, Cr %, Mo %, Nb %, Ti %, and Al % represent the contents (mass %) of the respective elements.

V and W are elements that are added in order to incrementally enhance the corrosion resistance of a duplex stainless steel.

V may be included at a content of 0.05% or more for the purpose of enhancing corrosion resistance. However, in the case where more than 0.5% of V is included, coarse V-based carbonitrides are generated, and toughness deteriorates. Therefore, the upper limit of the content of V is limited to 0.5%. In the case where V is included, the content of V is preferably in a range of 0.1% to 0.3%.

Similarly to Mo, W is an element that incrementally enhances the corrosion resistance of a stainless steel, and W has a large solid solubility compared to V. In the embodiment, W may be included at a content of 1.0% or less for the purpose of enhancing corrosion resistance. In the case where W is included, the content of W is preferably in a range of 0.05% to 0.5%.

That is, either one or both of the above-specified amounts of V and W may be included.

Co is an element effective for enhancing toughness and corrosion resistance of a steel, and Co may be selectively added. The content of Co is preferably in a range of 0.03% or more. In the case where more than 1.0% of Co is included, effects that are worth the costs are not exhibited since Co is an expensive element. Therefore, the upper limit of the content of Co is set to be 1.0%. In the case where Co is included, the content of Co is preferably in a range of 0.03% to 0.5%.

Furthermore, in order to improve hot workability, B, Ca, Mg, and REM may be included as necessary.

Here, REM refers to rare earth metals, and is one or more selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

B, Ca, Mg, and REM are all elements that improve the hot workability of a steel, and one or more may be added for that purpose. In the case where an excessive amount of B, Ca, Mg, or REM is added, hot workability and toughness degrade; and therefore, the upper limit thereof is set as follows. The upper limit of the content of each of B and Ca is 0.0050%. The upper limit of the content of Mg is 0.0030%. The upper limit of the content of REM is 0.10%. The content of each of B and C is preferably in a range of 0.0005% to 0.0030%. The content of Mg is preferably in a range of 0.0001% to 0.0015%. The content of REM is preferably in a range of 0.005% to 0.05%. Meanwhile, the content of REM refers to the sum of the contents of lanthanoid-based rare earth elements such as La, Ce, and the like.

Next, the reasons why the martensitic stainless steel material of the embodiment is limited will be described.

The martensitic stainless steel material of the embodiment has a composition of the above-described martensitic stainless steel of the embodiment, and has a metallic structure that meets the following requirements. Mechanical characteristics and strength of a base material can be secured by adjusting the phase fraction of the steel material.

A ferrite phase is soft, and, in the case where a certain amount of a ferrite phase is included, an excessive increase in strength is suppressed, and crystal grains are finely controlled through a dual-phase microstructure. Thereby, the toughness of the martensitic stainless steel material of the embodiment can be improved. A least 5% of a ferrite phase is required to improve the toughness. On the other hand, a ferrite phase is poor in toughness; and therefore, in the case where an excessive amount of a ferrite phase is included, the toughness of the martensitic stainless steel material of the embodiment degrades. In order to prevent the degradation of toughness, the fraction of a ferrite phase is set to be in a range of 30% or less. The fraction of a ferrite phase is preferably in a range of 5% to 20%.

This fraction of a ferrite phase is realized through the manufacturing conditions of the steel material in addition to the chemical composition, γ_(max) and γ_(pot). The above-described fraction range of a ferrite phase can be realized by selecting the manufacturing conditions from the manufacturing conditions of an ordinary stainless steel material depending on the chemical composition. For example, for rolling conditions, the heating temperature of hot rolling may be selected from 1150° C. to 1250° C. The finishing temperature of hot rolling may be selected from 950° C. to 700° C. In addition, in the case where a thermal treatment is carried out as necessary, the temperature of a quenching thermal treatment may be selected from 850° C. to 950° C. The temperature of a tempering thermal treatment may be selected from 550° C. to 750° C. In addition, the holding time of the temperature of the quenching thermal treatment is preferably in a range of approximately 5 minutes to 30 minutes. In addition, the holding time of the temperature of the tempering thermal treatment is preferably in a range of approximately 10 minutes to 1 hour.

In addition, a residual austenite phase is generated by remaining an austenite phase that is present at high temperatures in a non-transformation state. The residual austenite phase is soft; and therefore, the residual austenite phase enhances the toughness of the steel material. On the other hand, in the case where a residual austenite phase remains in excess, yield strength of the steel material decreases; and thereby, strength characteristics of the martensitic stainless steel material of the embodiment are impaired. Therefore, the upper limit of the fraction of a residual austenite phase is set to be 20%.

In order to control the amount of a residual austenite phase, it is necessary to control an Ms value (° C.) represented by the following formula 4. The chemical components are set so that the formula 4 becomes in a range of 200° C. or higher. In the case where the value of the formula 4 is lower than 200, the fraction of a residual austenite phase exceeds 20% which is the upper limit value specified in the embodiment. In addition, since the fraction of a residual austenite phase may be 0%, it is not necessary to set the upper limit of the Ms value (° C.) represented by the formula 4. In the composition range of the present embodiment, the Ms value can be set high in the permitted range. Meanwhile, the fraction of a residual austenite phase can be obtained through X-ray measurement. The amount of the residual austenite phase is preferably in a range of 3% to 15%.

Ms=1305−41.7%×(Cr %+Mo %+Cu %)−61×N %−33×Mn %−27.8×Si %−1667×(C %+N %)  Formula 4

Here, Cr %, Mo %, Cu %, Ni %, Mn %, Si %, C %, and N % represent the contents (mass %) of the respective elements.

In addition, the remainder other than the ferrite phase and the residual austenite phase is a martensite phase, and the sum of the fractions of three phases becomes 100%.

The yield strength of the martensitic stainless steel material of the embodiment is preferably in a range of 400 MPa to 800 MPa.

The embodiment relates to a martensitic stainless steel and a steel material which mainly include a martensite phase structure, and the embodiment has a high strength and excellent toughness. Therefore, in the case where the yield strength is less than 400 MPa, the value of the embodiment for applying to high-strength structure members, which is the object of the embodiment, is insufficient. On the other hand, in the case where the yield strength exceeds 800 MPa, even when a metallic structure is appropriately controlled, desired weld toughness cannot be secured. Therefore, the yield strength of the martensitic stainless steel material of the embodiment is preferably in a range of 400 MPa to 800 MPa.

EXAMPLES

Hereinafter, examples will be described.

Tables 1 to 4 show the chemical compositions of test specimen steels and the evaluation results of joint characteristics. These steels were manufactured by the following method. Steel ingots (50 kg) were manufactured through vacuum melting in a laboratory, and each of the steel ingots was cast so as to obtain rolling test specimens having dimensions of thickness 60 mm×width 110 mm×length 150 mm. After that, the rolling test specimens were hot-rolled such that the thickness became 12 mm.

The chemical compositions in Tables 1 to 3 are the analysis results of test specimens taken from the hot-rolled steel sheets.

Meanwhile, components other than the components described in Tables 1 to 3 (remainder) are Fe and inevitable impurity elements. In addition, in the components shown in Tables 1 to 3, the content of the component which is not described is an impurity level. In addition, REM in the tables refers to lanthanoid-based rare earth elements, and the content of REM shows the total content of the elements. In addition, Steel Nos. A to U are examples, and Steel Nos. V to AG are comparative examples.

Welding for evaluating the joint characteristics was carried out as follows.

The width center portion of the steel sheet was cut in a rolling length direction, and the end surface was cut so as to form a V-shape groove. Next, a joint was produced through two passes of welding under a heat input condition of 3.5 kJ/mm using welding rods for submerged arc welding of SUS329J3L and flux. From the welded joint, a Charpy test specimen was taken, and in the Charpy test specimen, a 2 mm V-shape groove was notched at a location 1 mm away from an interface between the weld metal and a heat-affected zone towards the heat-affected zone side. The test was carried out on two specimens at −20° C. for each of the test specimen steels. The average value of the obtained impact values was shown as the impact value 1 in Table 4.

Corrosion resistance was evaluated in the following manner.

A pitting potential measurement sample including the weld metal and the heat-affected zone was produced. Next, a silver-silver chloride electrode (SSE) was used as a reference electrode, and a pitting potential Vc′ 100 was measured in 3.5% NaCl at 30° C. according to JIS G0577. The results were shown in Table 4.

In the case where the impact value 1 was 35 J/cm² (=27 J) or more, the evaluation result was determined to be favorable. In addition, in the case where the pitting potential Vc′ 100 was 0.10 V or more which is the average pitting potential level of the base material of SUS430 steel, the corrosion resistance was determined to be favorable.

As a result, it was found that steels having the composition of the embodiment were all excellent in impact value 1 and corrosion resistance. In contrast, in Comparative Examples having compositions outside the range of the embodiment, steels were all poor in impact value 1 and corrosion resistance, which indicates the superiority of the steel of the embodiment.

In addition, Tables 5 to 8 show the manufacturing conditions, hot workability, metallic structures, and base material characteristics of the steel materials of the examples.

A rolling test specimen having dimensions of thickness 60 mm×width 110 mm×length 150 mm was heated to a predetermined hot rolling heating temperature, and then the rolling test specimen was subjected to a plurality of rolling such that the thickness became 12 mm. The temperatures at the final rolling were described as the hot rolling finishing temperature in Tables 5 and 6. The size of a cracked edge caused at the edge portion of the steel sheet obtained after the hot rolling was measured, and the hot workability was evaluated to be good in the case where the maximum cracked edge was 5 mm or less, the hot workability was evaluated to be bad in the case where the maximum cracked edge exceeded 5 mm, and evaluation results are shown in the “hot workability” column in Tables 5 and 6.

With regard to the obtained steel sheets or the steel sheets obtained by carrying out either one or both of a quenching thermal treatment and a tempering thermal treatment, metallic structures were investigated by the following method. A sheet thickness cross-section was etched so as to develop the microstructure. The metallic structure was observed using an optical microscope, and the area fraction of a ferrite phase was obtained through an image analysis. In addition, a test specimen having a measurement surface at a portion of ¼ of the sheet thickness and dimensions of 3 mm×23 mm×23 mm was produced, and the fraction of a residual austenite phase was quantified by the X-ray diffraction method. The results were shown in the “metallic structure” column in Tables 7 and 8.

Next, a tensile test and an impact test were carried out by the following method.

A tensile test specimen having a circular parallel portion with a diameter of 10 mm and a length of 60 mm was taken perpendicularly to the rolling direction. The test specimen was subjected to a tensile test, and the 0.2% yield strength was measured.

JIS No. 4 full-size Charpy test specimens having a 2 mm V groove were produced. The test was carried out on two specimens at −60° C. for each of the test specimen steels, and average impact values were measured. The average value of the obtained impact values was shown as the impact value 2.

In the case where the yield strength was 400 MPa or more, the yield strength was higher than that of an austenite stainless steel, and the yield strength was determined to be favorable. In the case where the impact value was 35 J/cm² (=27 J) or more, the impact value was determined to be favorable. As a result, it is found that Examples that correspond to the embodiment were all favorable in hot workability, base material strength, and toughness. In addition, from the results of Examples 34 to 37, it is found that the strength and the toughness of the base material can be secured without carrying out a quenching or a tempering thermal treatment. On the other hand, in Comparative Examples, the hot workability was insufficient, or either one of the base material yield strength or the impact value 2 was out of the desired value. From the results of Comparative Examples 39 and 40, it is found that, even for steels that meet the requirements regarding the chemical composition of the embodiment, in the case where the manufacturing conditions were not appropriate, and the metallic structure did not meet the requirements of the embodiment, desired characteristics were not obtained.

As is found from the above-described Examples and Comparative Examples, it became clear that a martensitic stainless steel with excellent weld characteristics can be obtained from the embodiment, and a martensitic stainless steel material with excellent characteristics in a base material and welded portions can be obtained by meeting the requirements regarding the metallic structure as well.

TABLE 1 Steel Chemical composition (mass %) No. Type C Si Mn P S Cu Ni Cr Mo Ti Al O N Others A Example 0.015 0.40 4.98 0.025 0.001 1.95 16.28 0.015 0.004 0.025 B 0.015 0.40 5.85 0.025 0.001 1.72 16.28 0.12 0.015 0.004 0.025 C 0.015 0.40 4.25 0.025 0.001 0.23 1.85 16.28 0.015 0.004 0.025 D 0.013 0.42 4.05 0.023 0.001 0.85 2.01 16.15 0.32 0.012 0.007 0.005 0.025 V: 0.05 E 0.012 0.40 4.23 0.024 0.002 0.45 2.12 16.62 0.005 0.012 0.003 0.028 V: 0.08, B: 0.0012 F 0.013 0.35 3.12 0.019 0.001 2.83 16.35 0.035 0.002 0.022 Nb: 0.03, Ca: 0.0025 G 0.019 0.38 4.53 0.013 0.001 0.36 1.52 15.26 0.23 0.022 0.003 0.021 V: 0.06, W: 0.32, Mg: 0.0018 H 0.012 0.43 4.95 0.024 0.002 0.45 1.98 16.23 0.035 0.014 0.003 0.022 Co: 0.12, REM: 0.040 I 0.005 0.44 4.89 0.022 0.001 0.45 2.13 16.35 0.015 0.003 0.026 Nb: 0.13, B: 0.0009, Ca: 0.0021 J 0.024 0.45 4.95 0.021 0.001 2.05 16.23 0.018 0.003 0.019 V: 0.05, Co: 0.05, Ca: 0.0015, Mg: 0.0008 K 0.013 0.46 3.52 0.021 0.001 1.35 2.12 17.25 0.013 0.003 0.024

TABLE 2 Steel Chemical composition (mass %) No. Type C Si Mn P S Cu Ni Cr Mo Ti Al O N Others L Example 0.013 0.40 4.07 0.015 0.001 0.82 1.99 16.20 0.31 0.005 0.008 0.005 0.025 W: 0.30 M 0.014 0.41 4.03 0.020 0.001 0.79 2.03 16.13 0.29 0.003 0.007 0.003 0.025 Nb: 0.03, W: 0.31 N 0.013 0.43 4.05 0.025 0.001 0.87 2.01 16.19 0.33 0.011 0.010 0.003 0.022 V: 0.12, Nb: 0.02, W: 0.29 O 0.012 0.42 4.04 0.019 0.001 0.85 2.02 16.15 0.35 0.004 0.009 0.004 0.025 V: 0.11, Nb: 0.03 P 0.013 0.39 4.02 0.023 0.002 0.83 1.95 16.22 0.32 0.013 0.007 0.004 0.021 Nb: 0.02, Co: 0.10 Q 0.013 0.41 3.98 0.026 0.001 0.85 2.00 16.18 0.31 0.003 0.007 0.005 0.026 W: 0.30, Co: 0.11 R 0.015 0.42 4.10 0.024 0.001 0.86 2.03 16.20 0.28 0.005 0.012 0.005 0.025 V: 0.12, W: 0.31, Co: 0.12 S 0.014 0.38 4.05 0.022 0.001 0.88 2.05 16.16 0.30 0.004 0.015 0.003 0.028 V: 0.11, Nb: 0.04, W: 0.29, Co: 0.10 T 0.013 0.40 4.06 0.021 0.002 0.82 1.98 16.06 0.34 0.008 0.006 0.004 0.020 B: 0.0020, Mg: 0.0011, REM: 0.035 U 0.012 0.42 4.08 0.023 0.001 0.85 1.97 16.14 0.33 0.012 0.007 0.005 0.025 B: 0.0018, Ca: 0.0017, Mg: 0.0010, REM: 0.037

TABLE 3 Steel Chemical composition (mass %) No. Type C Si Mn P S Cu Ni Cr Mo Ti Al O N V Comparative 0.015 0.40 5.02 0.025 0.001 1.25 15.45 0.015 0.004 0.010 W Example 0.013 0.45 2.48 0.024 0.001 2.05 16.21 0.005 0.005 0.038 X 0.015 0.40 6.52 0.025 0.001 1.95 16.28 0.011 0.004 0.024 Y 0.015 0.41 4.85 0.023 0.001 0.53 16.24 0.012 0.004 0.025 Z 0.016 0.35 4.53 0.021 0.001 2.01 14.52 0.012 0.004 0.025 AA 0.002 0.40 4.98 0.025 0.001 1.95 16.28 0.003 0.007 0.015 AB 0.013 0.42 4.89 0.021 0.001 1.98 16.25 0.09 0.015 0.003 0.032 AC 0.010 0.41 4.95 0.024 0.001 1.93 18.23 0.35 0.015 0.004 0.034 AD 0.017 0.45 4.98 0.025 0.002 1.95 16.28 0.015 0.004 0.045 AE 0.014 0.44 5.01 0.023 0.004 1.96 16.32 0.014 0.003 0.023 AF 0.021 0.42 4.86 0.022 0.002 1.97 18.32 0.013 0.003 0.024 AG 0.005 0.35 3.25 0.023 0.001 1.96 16.21 0.012 0.005 0.065

TABLE 4 Joint characteristics Steel γ_(pot) γ_(max) C + N Impact value 1 Corrosion resistance No. Type (mass %) (mass %) (mass %) (J/cm²) (V vs. SSE) A Example 77.3 94.2 0.040 55 0.15 B 80.3 93.5 0.040 70 0.13 C 70.3 88.8 0.040 35 0.14 D 73.3 93.7 0.038 45 0.19 E 75.6 93.3 0.040 55 0.13 F 71.7 97.9 0.035 35 0.14 G 71.5 93.0 0.040 35 0.15 H 75.1 96.3 0.034 50 0.16 I 77.1 96.7 0.031 60 0.14 J 80.2 97.1 0.043 55 0.14 K 70.9 86.9 0.037 75 0.19 L 73.3 92.8 0.038 40 0.20 M 74.7 94.6 0.039 40 0.20 N 70.6 91.6 0.035 45 0.20 O 72.9 93.0 0.037 40 0.19 P 68.1 89.6 0.034 45 0.19 Q 73.9 93.3 0.039 40 0.20 R 76.1 95.1 0.040 45 0.20 S 78.2 96.9 0.042 50 0.19 T 69.4 91.5 0.033 40 0.20 U 72.1 92.5 0.037 40 0.20 V Comparative 56.8 80.8 0.025 5 0.08 W Example 63.6 85.0 0.051 5 0.15 X 92.0 104.7 0.039 50 0.06 Y 47.9 61.1 0.040 5 0.08 Z 86.2 113.8 0.041 25 0.08 AA 60.6 84.6 0.017 5 0.12 AB 74.5 96.8 0.045 20 0.11 AC 64.8 68.9 0.044 25 0.24 AD 94.2 103.8 0.062 5 0.15 AE 74.9 92.4 0.037 25 0.07 AF 67.1 72.2 0.045 45 0.22 AG 86.2 98.4 0.070 5 0.15

TABLE 5 Steel Hot rolling Hot rolling Quenching Tempering material Steel heating finishing Hot thermal treatment thermal treatment No. Type No. temperature (° C.) temperature (° C.) workability temperature (° C.) temperature (° C.) 1 Example A 1200 900 good 900 None 2 B 1200 900 good 880 None 3 C 1200 900 good 900 None 4 D 1200 900 good 900 None 5 E 1200 900 good 900 None 6 F 1200 900 good 900 None 7 G 1200 900 good 900 None 8 H 1180 900 good 900 None 9 I 1180 900 good 900 None 10 J 1220 900 good 900 None 11 K 1220 900 good 900 None 12 L 1200 900 good 900 None 13 M 1200 900 good 900 None 14 N 1200 900 good 900 None 15 O 1200 900 good 900 None 16 P 1200 900 good 900 None 17 Q 1200 900 good 900 None 18 R 1200 900 good 900 None 19 S 1200 900 good 900 None 20 T 1200 900 good 900 None 21 U 1200 900 good 900 None

TABLE 6 Steel Hot rolling Hot rolling Quenching Tempering material Steel heating finishing Hot thermal treatment thermal treatment No. Type No. temperature (° C.) temperature (° C.) workability temperature (° C.) temperature (° C.) 22 Comparative V 1200 900 good 900 None 23 Example W 1200 900 good 900 None 24 X 1200 900 bad 900 None 25 Y 1200 900 good 900 None 26 Z 1200 900 bad 900 None 27 AA 1200 900 good 900 None 28 AB 1200 900 good 900 None 29 AC 1200 900 good 900 None 30 AD 1200 900 bad 900 None 31 AE 1200 900 bad 900 None 32 AF 1200 900 good 900 None 33 AG 1200 900 good 900 None 34 Example A 1200 900 good None None 35 A 1200 950 good None None 36 A 1200 900 good None 700 37 B 1200 900 good None 700 38 B 1200 900 good 880 700 39 Comparative C 1270 950 good None None 40 Example J 1100 800 bad None None

TABLE 7 Steel Metallic structure Base material characteristics material Steel Fraction of ferrite Fraction of residual Yield strength Impact value 2 No. Type No. phase (%) austenite phase (%) (MPa) (J/cm²) 1 Example A 8 2 600 120 2 B 6 6 520 160 3 C 15 0 680 85 4 D 10 5 640 135 5 E 8 7 460 145 6 F 8 0 720 75 7 G 10 0 710 70 8 H 10 4 630 135 9 I 10 6 540 140 10 J 10 5 520 130 11 K 10 15 420 185 12 L 10 5 640 130 13 M 10 10 660 130 14 N 10 5 650 130 15 O 10 5 660 135 16 P 15 5 650 140 17 Q 10 5 640 140 18 R 10 10 640 140 19 S 8 10 680 135 20 T 15 5 640 120 21 U 10 5 640 125

TABLE 8 Steel Metallic structure Base material characteristics material Steel Fraction of ferrite Fraction of residual Yield strength Impact value 2 No. Type No. phase (%) austenite phase (%) (MPa) (J/cm²) 22 Comparative V 25 0 530 15 23 Example W 35 0 660 5 24 X 3 15 420 145 25 Y 40 0 530 5 26 Z 3 0 780 30 27 AA 15 0 740 25 28 AB 15 0 600 30 29 AC 35 25 320 55 30 AD 2 10 850 25 31 AE 10 1 710 65 32 AF 35 25 320 140 33 AG 5 0 900 5 34 Example A 10 2 620 110 35 A 15 3 580 130 36 A 12 0 720 120 37 B 10 0 670 150 38 B 10 0 660 140 39 Comparative C 35 0 580 10 40 Example J 4 2 840 30

INDUSTRIAL APPLICABILITY

In accordance with the embodiment of the invention, it is possible to provide an economic martensitic stainless steel material having favorable weld characteristics and a small content of Ni. Therefore, it is possible to provide a low-cost high-strength steel material that can be applied to large-scale structures. In addition, since long-term thermal treatments that were required in the related art can be skipped, the embodiment of the invention can improve mass productivity and greatly contribute to the industry. 

1. A martensitic stainless steel with excellent weld characteristics comprising, in terms of percent by mass: C: 0.003% to 0.03%; Si: 0.01% to 1.0%; Mn: 3.0% to 6.0%; P: 0.05% or less; S: 0.003% or less; Ni: 1.0% to 3.0%; Cr: 15.0% to 18.0%; Mo: 0% to 1.0%; Cu: 0% to 2.0%; Ti: 0% to 0.05%; N: 0.05% or less; Al: 0.001% to 0.1%; and O: 0.005% or less, with a remainder being Fe and inevitable impurities, wherein a total amount of C and N is in a range of 0.060% or less, γ_(max) represented by the formula 1 is in a range of 80 or more, and γ_(pot) represented by the formula 2 is in a range of 60 to 90, γ_(max)=420×C %+470×N %+23×Ni %+9×Cu %+7×Mn %−11.5×Cr %−11.5×Si %−52×Al %+189  Formula 1 γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−74.4×Ti %−37.2×Al %+63.2  Formula 2 in which C %, N %, Ni %, Cu %, Mn %, Cr %, Si %, Al %, Mo %, and Ti % represent the contents (mass %) of the respective elements.
 2. The martensitic stainless steel with excellent weld characteristics according to claim 1, wherein the martensitic stainless steel further comprises one or more selected from Nb, V: 0.5% or less, W: 1.0% or less, Co: 1.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less, and if Nb is included, γ_(pot) that is calculated by the formula 3 instead of the formula 2 is in a range of 60 to 90, γ_(pot)=700×C %+800×N %+10×(Mn %+Cu %)+20×Ni %−9.3×Si %−6.2×Cr %−9.3×Mo %−3.1×Nb %−74.4×Ti %−37.2×Al %+63.2  Formula 3 in which C %, N %, Mn %, Cu %, Ni %, Si %, Cr %, Mo %, Nb %, Ti %, and Al % represent the contents (mass %) of the respective elements. 3-5. (canceled)
 6. A martensitic stainless steel material, wherein the martensitic stainless steel material has a composition according to claim 1, and the martensitic stainless steel material has a structure comprising 5% to 30% of a ferrite phase and 0% to 20% of a residual austenite phase with a remainder being a martensite phase.
 7. The martensitic stainless steel material according to claim 6, wherein a yield strength is in a range of 400 MPa to 800 MPa.
 8. A martensitic stainless steel material, wherein the martensitic stainless steel material has a composition according to claim 2, and the martensitic stainless steel material has a structure comprising 5% to 30% of a ferrite phase and 0% to 20% of a residual austenite phase with a remainder being a martensite phase.
 9. The martensitic stainless steel material according to claim 8, wherein a yield strength is in a range of 400 MPa to 800 MPa. 